Hemispheric Modeling

The expansion of detailed regional models to the hemispheric scale provides opportunities for consistently representing various atmospheric processes at the disparate scales. EPA is currently adpating the Community Multiscale Air Quality (CMAQ) modeling system to simulate O3, particulate matter, and related precursor distributions over the northern hemisphere (Mathur et al., 2012ab).

The horizontal domain, set on a polar stereographic projection, is discretized using grid cells with a 108 km resolution, while the vertical extent ranging from the surface to 50mb was discretized with 44 layers of variable thickness, with a 20 m deep lowest layer. 3-D meteorological fields were derived from the Weather Research and Forecasting (WRF) modeling system operating on the exact same projection and grid configuration as CMAQ. Emissions of NOx, SO2, CO, volatile organic compounds and particulate matter are derived from a variety of global emission inventories.

Model simulations have been conducted for the entire 2006 calendar year, and model estimates of the 3-D distribution of gas- and aerosol-phase species have been compared with measurements from the INTEX-B field campaign, the INTEX Ozonesonde Network Study (IONS), surface network data across the U.S., as well as aerosol optical depth retrievals from AERONET and MODIS (Mathur et al., 2012a).

Additionally, EPA is examing the hemispheric model's ability to provide spatially and temporally varying and consistent lateral boundary conditions (LBCs) to limited area finer resolution calculations by nesting a 12km grid over the Continental U.S. within the hemispheric domain (Mathur et al., 2012b). These initial applications are demonstrating the feasibility of extending the applicability of the CMAQ modeling system to hemispheric scales to provide a conceptual framework to examine interactions between atmospheric processes occurring at various spatial and temporal scales in a consistent manner.